Generally, a drug administered for the purpose of medical treatment or diagnosis once goes through the systemic blood circulation and then takes the method of absorption, distribution, metabolism, excretion and the like. In the method of absorption and distribution, the drug moves in the flow of the blood and migrates in each of the intravascular, interstice and intracellular spaces by diffusion and transportation of a free drug in a state not bound to protein, and finally the drug arrives at the active site of the target. When the migration of the drug reaches a steady state, the concentration of the free drug in each space becomes uniform, and the whole pattern of the drug concentration depends on the binding level to proteins and the like.
In this way, the drug exists in vivo partially and reversibly binding to a biopolymer such as a plasma protein or the like in accordance with the property thereof. Since the drugs permeable through capillary wall or cell membrane are unbound drugs in general, the drugs which may act as active ingredients are free drugs being unbound to plasma protein, etc. and the migration to the active site of the target is greatly influenced by the binding level of the drugs to the plasma protein, etc. The binding between the plasma protein and the drug affects the distribution to focus part on which the drug should act and excretion thereof, therefore, it is required in the development of a drug to investigate whether the drug binds to the plasma protein or not and to measure the binding level when they bind together.
WO00/78352A describes a method of administrating a drug comprising administrating the first drug having binding affinity to a plasma protein and administrating the second drug having binding affinity to the plasma protein in common with the first drug and thereby making it possible to regulate the binding of the first drug to the plasma protein as well as a pharmaceutical preparation therefor. That is, this WO publication describes that the administration of the second drug simultaneously with or before or after the administration of the first drug makes it possible to control the binding of the first drug to the plasma protein and to increase or decrease the concentration of the first drug freed in the blood.
For example, 99m-technetium labeled mercaptoacetylglycylglycylglycine (99mTc-MAG3) is efficiently excreted into urine by renal tubular secretion in the kidney, and is a widely used radioactive pharmaceutical substance for use in vivo for the purpose of diagnosis of kidney and urinary tract diseases. It is known that about 90% of 99mTc-MAG3 binds to plasma protein when used in the concentration of a diagnostic agent. WO00/78352A describes that when 99mTc-MAG3 is used as the first drug, binding between 99mTc-MAG3 and plasma protein is controlled by the administration of bucolome, cefazolin, valproic acid, etc., which is the second drug, the concentration of the free 99mTc-MAG3 can be raised (displacement effect) and as a result 99mTc-MAG3 can be more efficiently excreted into urine.
Japanese Patent Application No. 2002-267010 by the present inventors describes that if a preparation containing an active ingredient which has binding affinity to a plasma protein is administered simultaneously with or before or after the administration of another preparation containing an amino acid which has binding affinity to the plasma protein in common with the active ingredient, competitive displacement occurs at the binding site and the concentration of the free active ingredient increases (displacement effect) and therefore it can be expected to attain higher drug activity rather than administrating the preparation containing the active ingredient alone to the patient. On the contrary, when the binding of an active ingredient to the plasma protein is increased by the action of the preparation containing amino acid, the concentration of the free active ingredient decreases (reduction effect of free drug concentration) and therefore it can be expected to attain continuous appearance of pharmacological effect due to decreased clearance resulting from the low concentration of the free active ingredient in the blood maintained for a longer period of time.
In the meantime, it is expected that the displacement effect by the preparation containing the second drug or an amino acid as mentioned above may vary depending on the first drugs and the living body to be administered. Typical plasma protein to which a drug binds includes albumin and acid glycoprotein, and it has been revealed that each of them has two or more binding sites, respectively. For example, when the binding site of the first drug and the binding sites of the second drug are the same, the concentration of the first drug freed is expected to change a lot due to the displacement effects of the second drug, while the binding sites differ, the change of the concentration of the first drug freed due to this displacement effect is expected to be small. For this reason, when the displacement effect by the second drug is anticipated, it is necessary to investigate the binding site(s) on the plasma protein to the first and second drugs beforehand. This is because if it can be known beforehand to which site of two or more drug binding sites on plasma protein the drug binds, prediction of pharmacokinetics of the drug will become easier.
On the other hand, when there are mutation and the like in albumin and acid glycoprotein (typical plasma proteins) themselves, the above-mentioned displacement effect may be significantly influenced. If change arises in the proteinic binding site, it will cause changes in the binding to a drug.
However, no simple method for determining the binding site and the amount of binding of a drug and plasma protein has not been developed until now.
In order to determine the displacement effect of a drug, as is described in WO00/78352A, the first drug labeled with a radioactive nuclide and the second drug for displacement is mixed with plasma, the radioactivity of the whole mixed solution is measured and the radioactivity of the filtrate containing the drug which has not bound to the protein is measured after carrying out ultrafiltration just to compare the both. However, by this method, there were problems that whenever the first drug and the second drug were changed, the displacement effect of the second drug should be determined, and that, for this reason, so much blood had to be sampled from the critical patient for measurement, and, in addition, the measurement takes labor and time.